The objectives of this proposed research project are to determine the extent of DNA repair synthesis in precancerous rat liver and to determine the feasibility of detecting environmental carcinogens by their ability to provoke DNA turnover and/or DNA repair synthesis in a live rat. Rat liver DNA will be prelabelled with thymidine-methyl-3H (TdR-3H) or 5-bromodeoxyuridine-C14 (BUdR-14C) during liver regeneration following partial hepatectomy. The animals will be placed on a carcinogen diet. Rats prelabelled with BUdR will be injected with TdR- 3H, heavy and light DNA strands will be separated in alkaine cesium chloride gradiants and 3H in the heavy strand would be an indication of repair synthesis. The DNA of rats prelabelled with TdR-3H will be employed in alkaline sucrose gradient centrifugation studies to monitor single strand breaks in DNA. Enzymes involved in DNA repair synthesis (DNA ligase, exonuclease and endonuclease) will be assayed at various times after the start of carcinogen feeding. Rats with prelabelled DNA will be employed for studies on DNA turnover during carcinogen ingestion. At various times after the start of carcinoen feeding the covalent binding of metabolites of the carcinogen to hepatic DNA will be determined. The total amount of binding will be measured and the binding to specific bases will be studied by using liquid chromatography to separate the bases. In addition, the incorporation of thymine as compared to thymidine into the DNA of Novikoff Hepatoma Cells in tissue culture will be examined to determine whether or not different pools of thymidine nucleotides (one for DNA repair and one for DNA replication) exist in a mammalian cell, a recent study has indicated this to be the case in bacteria.